A chemical compound, or a mixture of compounds, may exist in different solid forms, each of which has a characteristic free energy at a given temperature. A compound is a substance composed of atoms or ions in chemical combination. A compound will usually include atoms or ions of two or more elements, but as used herein, may include substances composed of one element. The “form” of a compound or mixture refers to its arrangement of molecules or atoms in the solid or semi-solid state. Different forms of a compound or mixture may be distinguished by their x-ray diffraction patterns as well as other suitable means. A compound or mixture may be arranged in a crystalline state, where the molecules exist in fixed conformations and are arranged in a regular way. A compound or mixture may exist in different possible crystalline forms. Further, a compound or mixture may have different crystalline forms that correspond to different free energy levels. A chemical compound or mixture may be amorphous, meaning that it is not characterized by a regular arrangement of molecules, which tends to indicate a relatively high free energy state. The same compound or mixture may exhibit different properties depending upon which form it is in (such as amorphous or crystalline, or such as one of several different crystalline forms).
A compound or mixture will have a most stable solid form at a given temperature (that is, its lowest free energy form at that temperature), and may have less stable forms, which are referred to herein as high free energy forms, or as metastable forms in some contexts. For example, if a compound crystallizes in a stable crystal form that is the most stable form that can be found, then any other form that is found may be considered a high free energy form, in that it has higher free energy than the most stable form. Such forms are metastable thermodynamically in that they are stable enough to be found in solid form, at least for some period of time.
Past attempts to generate high free energy forms involved flash evaporations, cooling under different conditions, and/or the addition of seeds of solid material. However, some materials strongly resist the generation of high free energy forms, and previous attempts to generate high free energy forms of such materials have not been satisfactory. For example, some systems, such as glycogen, do not form high free energy forms unless there is a change in pH or temperature. However, for a variety of reasons, it may not be desirable to alter pH, temperature or other conditions when attempting to generate high free energy forms.
When a compound has different solid or crystalline forms, the different forms are frequently referred to as polymorphs of the compound. A “polymorphic” compound as used herein means a compound having more than one solid form. For example, a polymorphic compound may have different forms of its crystalline structure, or different forms based upon hydration, or it may have a crystalline form and an amorphous form.
There are several reasons why it may be desirable to search for different polymorph forms, including different free energy forms, of a compound or mixture. Different free energy forms of the same compound or mixture may exhibit different properties. As a result, different free energy forms, including different crystalline forms, of a compound or mixture may have greater or lesser efficacy for a particular application.
One or more solid forms may be generated by crystallization of the sample. Among the phenomena in crystallization are nucleation and growth. Crystal nucleation is the formation of an ordered solid phase from liquids, supersaturated solutions, saturated vapors, or amorphous phases. Crystals may originate on a minute trace of a foreign substance (either impurities or container walls) acting as a nucleation site. Since nucleation may set the character of the crystallization process, the identity of the foreign substance is an important parameter. The presence of “seeds” of other crystalline compounds in a crystallization environment can be beneficial, detrimental, or both, but in any event, usually has an influence. Growth is the enlargement of crystals caused by deposition of molecules on an existing surface.
Typically, a solid to be crystallized is present in a solution at, above, or below its saturation point at a given temperature. Crystallization is initiated or facilitated by removing solvent, changing temperature, and/or adding an antisolvent. The solvent may be removed by evaporation or other means. Alternatively, the temperature of the solution is changed, resulting in crystallization. Eventually the solution reaches a point where crystals will grow.
During a crystallization process, a specific chemical substance may crystallize into different forms. For example, ammonium nitrate exhibits different crystal forms depending on the temperature. Below −18° C., ammonium nitrate exhibits a tetragonal crystal form, and above that temperature, it exhibits an orthorhombic form. Above 32.3° C., ammonium nitrate exhibits a different type of orthorhombic form, and above 84.2° C. it exhibits a trigonal form. Above 125.2° C., ammonium nitrate exhibits a cubic crystal form, and at 169.6° C. ammonium nitrate will liquefy at atmospheric pressure. At a given temperature the lowest free energy form frequently is preferentially formed and the others have relatively higher free energy. Transitions from one polymorph form, pseudopolymorph form, or amorphous form to another form may be accompanied by other physical or chemical changes. The different forms of ammonium nitrate arise from the different packing arrangements into which the molecules crystallize at different temperatures. Some compounds may have different colors that indicate different free energy forms. For example, the compound 5-methyl-2-[(2-nitrophenyl)amino]-3-thiophenecarbonitrile exhibits different colors depending on which solid form it is in.
A specific solid form may be more preferable than another solid form. For example, one polymorph may have a more desirable color or greater hardness or disperse in water more easily than another polymorph. Often one polymorph form is more stable than another form. For example, at 80° C., one orthorhombic form of ammonium nitrate is more stable than the trigonal form. One approach to keeping a less stable polymorph from transforming to a more stable but less desirable polymorph form requires the use of an additive to block rearrangement of the crystal structure leading to the undesired form.
It is known to generate crystalline samples in capillary tubes. For example, U.S. Pat. No. 5,997,636 discusses a method for growing crystals within a capillary tube. The patent primarily discloses crystallizing proteins, and the patent does not disclose the relative free energy of the proteins formed or that different forms of proteins were formed.
As another example, D. Amaro-González et al., “Gas Antisolvent Crystallization Of Organic Salts From Aqueous Solution”, Journal Of Supercritical Fluids, 17 (2000) 249–258, discloses results of crystallization of lobenzarit, including crystallizations in capillaries. Lobenzarit is an anti-arthritic agent. Amaro-González et al. state that particle size and agglomeration varied depending on the size of the capillary, that it is shown that the size distribution and morphology can be controlled using different capillary diameters, and that it is possible to obtain individual crystals without agglomeration.
The reference does not disclose that different crystal forms (arrangements at the molecular or atomic level in the solid) were produced. A different particle size or shape does not imply a different crystal form since a solid form can crystallize into many different shapes. For example, snowflakes may comprise a single crystal form having many different crystal shapes. The reference also does not disclose that a plurality of different forms may be generated in a plurality of capillary tubes having the same size and shape.
As another example, U.S. Pat. No. 4,290,835 discloses that various processes are known for growing crystalline bodies and that one such process, referred to as the “capillary die process,” generally uses a capillary die or forming member from which the crystalline body can be grown.
As yet another example, U.S. Pat. No. 4,295,857 discusses a process of the crystalline precipitation of a chromogen within a capillary. A chromogen is an aromatic compound having a chemical grouping, the chromophore, which gives color to the compound. The patent discloses that it is desirable to prepare a reagent component or mixture in a capillary because it allows for handling of very small amounts of substance and it excludes or at least reduces the danger of errors in dosing of the substance, such as in clinical chemical applications. The examples disclosed the crystallization of 4-aminophenazone in numerous capillary tubes, such as in wheels of five to ten capillary tubes.
As a final example, U.S. Pat. No. 5,363,797 discloses a method for producing a single organic crystal in a capillary tube.
None of the foregoing references disclose that a high free energy form of a compound or mixture may be obtained by solidification or crystallization within a capillary tube. Indeed, none of those references are directed to generating or searching for different free energy forms of a compound or mixture, and none disclose that a high free energy form may be isolated, analyzed and/or stabilized within a capillary tube.
There are several factors that discourage the use of capillary tubes for solidifying compounds or mixtures. One factor is that capillary tubes are more difficult to work with than other containers. Another factor is that there has been no general recognition that the use of capillary tubes may affect reactions or lead to compositional or chemical differences. Thus, since it was believed that the same forms and reactions could be done in other containers, it is believed that capillary tubes have not been used to search for and generate high free energy forms.